Jet mill micronizers are commonly used to reduce the particle size of friable material to the micron range. Typical jet mill micronizers feed the friable material into a vortex created by injection of a fluid such as compressed air, gas or steam through a nozzle into the micronizer. The vortex entrains the friable material and accelerates it to a high speed. Subsequent particle on particle impacts within the micronizer create increasingly smaller particles, with particles of the desired size ultimately moving to the center of the micronizer where they exit through a vortex finder.
The efficiency of the micronizer is dictated by the ability to properly entrain the friable material within the jet stream created by the injected gas. Over the years, the industry has attempted to improve the entrainment of the particles through changes in nozzle design as well as through recirculation devices incorporated into the micronizer. While such efforts have met with limited success, they frequently rely upon complicated designs subject to wear and increased maintenance.
One attempt to improve the efficiency of a micronizer resulted in the development and use of the now standard convergent-divergent nozzles. Converging-diverging nozzles generate extremely high velocity gaseous streams commonly achieving supersonic velocities. However, because the gaseous streams expand within the nozzle, entrainment of particles within the resulting jet is difficult. Thus, the benefits of the supersonic velocity are not generally imparted to the friable material.
High pressure steam is commonly used to generate the micronizing jet when milling titanium dioxide particles to pigmentary size. In view of the energy costs associated with steam generation, improved entrainment efficiencies can lead to significant cost savings during the TiO2 pigment manufacturing process. The quantity of steam used during the TiO2 micronization process, for example, is typically quite substantial, generally varying between about 0.5 to greater than two tons per ton of pigment.
In view of the significant energy costs associated with steam jet mills, it would be desirable to provide an improved jet nozzle which enhances entrainment of particles to be milled. Preferably, such improvements would be provided without significant design changes to the micronizer. Further, it would be even more beneficial if the changes enabling the improved operations of the micronizer could be readily retrofitted to existing units. The current invention, as described herein, provides for each of the above needs through an improved micronizer jet nozzle.